The invention relates to a process and the double-sided chemo-mechanical polishing of semiconductor wafers, as well as to the semiconductor wafers obtainable thereby. More particularly, the invention relates to such a process and apparatus, and semiconductor wafers obtainable thereby, wherein the front and rear sides of the wafers are polished differently under the simultaneous action of an upper and a lower moving polishing surface, to which a polishing agent is applied.
Owing, above all, to the increasing demands on the flatness of wafers with large diameters, in the chemo-mechanical polishing of semiconductor wafers, double-sided polishing processes are gaining ever more in importance by comparison with the conventional single-sided polishing. In the case of double-sided polishing processes, the front and rear sides of the wafer are acted upon by simultaneously moving polishing surfaces that are configured in the known polishing machines as counter-rotating, flat polishing plates. These plates are covered with polishing cloths, to which a polishing agent is applied. In this process, the wafers are generally held in the openings of flat rotor disks, through which they experience an additional rotary movement between the upper and lower polishing plates. Double-sided polishing processes are described in the technical literature and known to the person skilled in the art; for example, technical machine aspects are described in the article by E. Mendel and J. R. Hause entitled "Multiple Wafer Free Polishing--Part I, Machine Concept," dated Apr. 10, 1980, which appeared as IBM Technical Report TR 22.2342, and technical process aspects of double-sided polishing are described in the article by E. Mendel and J. S. Basi entitled "Multiple Wafer Free Polishing--Part 2 Process," dated Apr. 10, 1980, which appeared as IBM Technical Report TR 22.2342.
The main advantage of the wafers obtained with the aid of the double-sided polishing process resides in their improved flatness by comparison with single-sided polished wafers. In this regard, the standard processes deliver wafers whose front and rear sides are polished in the same way and cannot be distinguished. In the fabrication of electronic components, however, the processes of most manufacturers are adapted to single-sided polished wafers, it often being the case that a further gettering damage, i.e., a surface layer that is subjected to a mechanical stress, e.g., provided with scratches, is required on the rear side. As a rule, such getter layers are applied before polishing. However, as a consequence of the strain arising during cementing, the surface of single-sided polished wafers has a certain waviness, which can be recognized, for example, with the so-called "Magic Mirror" (cf. in this connection, e.g., U.S. Pat. No. 4,547,073).
For these reasons, great interest attaches to a polishing process in which the advantages of double-sided polishing with respect to the excellent flatness can be combined with the product characteristics of single-sided polishing. In the above-mentioned article concerning the machine concept, the possibility of obtaining variously high abrasion rates and, thus, variously polished front and rear sides of the wafers, by the simultaneous use of various polishing cloths on the upper and lower polishing plates is noted. However, in the normal course of the polishing process, in which no direct contact occurs between the polishing cloth and the wafer, the differences thereby achievable between the two wafer surfaces are small. On the other hand, such a direct contact must be avoided, since it is extremely damaging for the course of the process, so that it is fundamentally scarcely possible for the result of the polishing process to be influenced in the case of this mode of procedure.